Not Applicable
Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to electrical energy storage devices, and more particularly to a high power liquid electrolyte battery providing electrodes with large reaction surface areas, a long cycling life and a production method for active materials using conductive polymeric matrixes.
2. Description of the Background Art
A lead-acid storage battery remains the battery of choice for traditional uses such as starting an automobile, providing emergency lighting, power for an electric vehicle or as a storage buffer for a solar-electric system. Such batteries may be charged, periodically, by a generator driven by an engine or by some other source of electrical energy. The electrical production and cycling demands that are placed on modern automotive batteries in particular are greater than ever before. These demands, among other factors, result in a reduction in the service life of the battery.
Lead-acid batteries typically comprise a series of cells consisting essentially of a positive plate containing lead or lead oxide and a negative plate made of a material such as sponge lead. The positive and negative plates of each cell are oriented vertically, connected in parallel and submerged in an electrolyte solution such as dilute sulfuric acid.
Perhaps the most common type of positive plate found in current lead-acid batteries is the pasted plate. During manufacture of the pasted plate type grid plate, lead oxide powder is mixed with water and sulfuric acid to form a basic sulfate complex, which is shaped into a plate. A porous interconnected structure is produced from the basic sulfate complexes during the steaming, curing and drying of the plates. The building blocks of the skeleton are basic sulfate crystals of approximately 20 xcexcm to 50 xcexcm in width and 100 xcexcm in length. The skeleton provides an electrical conducting matrix including voids and channels for acid diffusion and provides an active surface for chemical and/or electrochemical reactions.
The morphology of the active material in green plates has been found to be an essential factor in the overall performance and service life of the battery. It is critical to create and maintain a strong and stable porous interlocked skeletal structure in the plates to achieve good performance and service life in the battery. For example, battery plates made up of tetra-basic sulfate crystals that have a high paste density with low porosity will result in a plate that has a long cycling life but low active material utilization in electrical performance.
In contrast, battery plates manufactured from low density paste with small sulfate crystals will allow a larger active material utilization but reduced cycling life due to reduced physical contact between active material crystals.
After the green plates are processed and assembled into a battery, sulfuric acid is added to the battery to react chemically with the paste materials to form lead sulfate, (PbSO4). This process is called pickling or sulfation. The pickling reaction typically includes the conversion of the PbO component in free oxides (PbO) and basic lead sulfate complexes (PbO.PbSO4, monobasic lead sulfate; 3PbO.PbSO4.H2O, tribasic lead sulfate; 4PbO.PbSO4.H2O, tetrabasic lead sulfate) to PbSO4 as follows:
PbO+H2SO4xe2x86x92PbSO4+H2O
PbO.PbSO4+H2SO4xe2x86x922PbSO4+H2O
3PbO.PbSO4.H2O+3H2SO4xe2x86x924PbSO4+4H2O
4PbO.PbSO4+4H2SO4xe2x86x925PbSO4+4H2O
After the pickling step, PbSO4 is electrochemically converted to lead dioxide (PbO2) in the positive plate and sponge lead (Pb) in the negative plate formation process. The morphology of the formed active material depends on the original plate microstructure and the formation conditions (e.g. Input current, filling acid concentration, formation temperature and formation time). The mass-transfer behavior of the electrolytic species (H+, H2SO4, and H2O), to and from the surface of the active materials is another essential factor that controls the power and energy capabilities of a battery. The mass-transfer rate is affected by the macro and micro-porosity in the paste, the plate thickness, acid concentration and application conditions.
During the formation and application cycles of lead-acid batteries, a layer of oxide, approximately 0.001 inches to approximately 0.030 inches thick, is typically formed on the surface of the positive grid due to corrosion. The composition of the corrosion layer is responsible for electrical conductivity. The positive polarization oxidation of the lead (Pb) grid in sulfuric electrolyte solution produces a corrosion product consisting primarily of PbO2 and far lesser amounts of PbO and PbOx. The corrosion layer is typically composed of a matrix of lead (IV) ions (Pb+4), and oxygen ions (Oxe2x88x922).
During discharge, the positive active material PbO2 is converted to PbSO4, which is an electronic insulator. Thus, the grid is thermodynamically unstable and alternatively forms PbO2 on the surface of the grid during charging and PbSO4 during discharge.
In addition to the formation of PbSO4 during discharge, the volume of the active material changes which is detrimental to the structural integrity of the positive and negative electrodes. For example, an increase of as much as 200% in the volume of the positive active materials and 260% in the negative active materials has been observed during discharge. Decreases in volume have also been observed during the charging cycle. Consequently, the repetitive expansion and contraction of the active materials often leads to the loss of crystal linkages in the skeletons eventually causing failure in the battery.
Another consequence of changes in volume in the active material is the loss of adhesion of the corrosion layer with the grid leading to the shedding of active materials that accumulate in the bottom of the battery casing. Over time the collected materials may accumulate on the bottom of the battery to a depth that allows a short circuit resulting in a dead cell and loss of battery life.
The aforementioned problems are accentuated in battery designs utilizing very thin plates to reduce the overall weight of the battery for use in aircraft and electric cars and the like. While thinner lightweight plates reduce the weight of the battery, the structure of the cell plates may not be sufficiently strong to prevent structural failure during the normal use of the battery.
Therefore, a need exists for a lead-acid battery that will maintain strong and stable skeletal structures during multiple charging cycles, provide a high reaction surface area as well as increased electronic conductivity of the active materials that can be easily and inexpensively fabricated. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previous battery designs.
The present invention provides a battery plate and a method of manufacturing a battery plate with a surface that has very fine crystals in the order of approximately 100 nm to approximately 1 xcexcm deposited within a conductive polymeric skeleton or matrix. The active polymeric skeleton can provide such critical functions as an electron and ion conductor, voids and channels for acid electrolyte diffusion and a structural support for active nanoscale crystals of PbSO4 or basic lead sulfate complex. The conductive polymeric skeleton can also provide a very high surface area for the chemical and electrochemical reactions that take place during charge and discharge. The skeletal structure also allows a substantial increase in the utilization of the active material over the prior art while reducing the weight of the plate. In addition, the positive grid oxidization should be minimized due to the presence of the polymer film on the grid surface.
According to an aspect of the invention, a layer of conductive polymer is polymerized on the surface of a battery plate that is preferably cross-linked and highly conjugated in structure. The conductive polymers are preferably selected from the family of polyaniline compounds (PAN) and their derivatives. Polyaniline is a preferred polymer because it is electroactive in acidic media and is stable in sulfuric acid solutions over a wide range of applied potentials. The preferred form of PAN is sulfonated.
The active materials can be applied to a variety of cell designs including bipolar configuration, flat plates, spiral wounds or thin films. Depending on the application requirements, the electrodes can be produced in many different shapes and thicknesses.
An object of the invention is to provide lead acid battery plates that have a high percentage utilization of grid active material while maintaining strong skeletal structures in the plate.
Another object of the invention is to provide a lead acid battery that can be deeply discharged and have a long cycling capability.
Yet another object of the invention is to provide a battery plate that has conductive polymers on the surface that minimize positive grid oxidation and have a high reaction surface area.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.